Lighting solutions based on energy-efficient light-emitting diodes (LEDs) are becoming more widespread. However, a relatively high failure rate of LED panels (strings of LEDs arranged on a heatsink) used in some LED lighting solutions has been observed. Current surges have been identified as the cause of many of the failures. It is known that a common-mode current surge is facilitated by certain types of LED driver in which a “Y-capacitor” is arranged across the primary and secondary windings of the driver transformer. Such a transformer is required for providing the necessary DC operating voltage and current for the LEDs, and a Y-capacitor is used to reduce electromagnetic interference (EMI). The occurrence of a current surge at the driver cannot be reliably prevented, and such a current surge can pass through the Y-capacitor and enter the LED panel. The usual LED panel design further facilitates surge current damage, since this generally includes unavoidable parasitic capacitances between thermal pads of the LEDs and a ground or earth connection. During a current surge, these capacitors become charged, and since the LEDs effectively lie in the surge current path because the thermal pads are electrically connected to the LEDs, the peak current also passes through the LEDs and may destroy them.
It is known that the usual Y-capacitor between primary and secondary sides of the driver provides a path for a surge current from the primary side to the secondary side. One way of avoiding LED damage is to split or actually remove the Y-capacitor from between the primary and secondary sides of the driver. However, the Y-capacitor is part of the driver's EMI damping system, and splitting or even removing this capacitor introduces a severe penalty to the EMI performance of the driver. A solution with split Y-capacitor can result in severe increase in EMI, for example by a factor of ten.
Another solution is to bypass each individual LED by a capacitor, so that a surge current will take the path through the bypass capacitor rather than through the LED. However, adding such bypass capacitors to an LED board requires custom LED board design, which may be prohibitively expensive or commercially unrealistic.
Both of the known solutions provide a good level of surge protection, but are associated with penalties that have a negative impact on other performance criteria of a luminaire or the cost of realization.
Therefore, it is an object of the invention to provide an LED lighting circuit that avoids the problems described above.